1. Field of the Invention
This invention relates to an aqueous emulsion which crosslinks by a Michael addition reaction, yielding a crosslinked silicon modified organic composition upon the removal of water and to methods of preparing such emulsions.
2. Background Information
Environmental concerns are leading to changes in the technology for coatings. Of particular concern is the amount of volatile organic compounds (VOC) released into the atmosphere from coatings. In water-based coatings, volatile solvents are used to promote coalescence and film formation of latex particles. This is usually accomplished by preparing a dispersed polymer or copolymer having a Tg above room temperature and plasticizing it with a volatile solvent to effectively lower its Tg so it can form a film at room temperature. Evaporation of the solvent after film formation leaves a polymer that was in effect applied below its actual Tg; hence no external heating is required to achieve film formation. Although this concept works quite well, it is becoming increasingly less popular as the levels of VOC in coatings are being reduced significantly due to more stringent legislation throughout the world.
One method for overcoming this use of solvent is to crosslink emulsion polymers, preferably after removal of water. Crosslinking leads to increased mechanical properties of a polymer film and under the proper conditions it can lead to a reduction in the amounts of solvents used for coalescence and film formation. However, the choices available for crosslinking water-based coatings after removal of water are limited.
U.S. Pat. No. 3,729,438 discloses emulsion polymers containing siloxane functionality formed from copolymers of vinyl acetate and a vinyl hydrolyzable silane, such as for example, 3-methacryloxypropyltrimethoxysilane or vinyltrimethoxysilane (VTMS). These silanol modified organic polymers are disclosed as capable of post-crosslinking by means of the hydrolyzable siloxane functionality. It was attempted to retard premature condensation crosslinking through pH control of the aqueous emulsion within the range of pH 3.5 to 6.
U.S. Patent Application, Liles, et al, "Aqueous Emulsions of Functionalized Organic Polymers," Attorney Docket No. DC4398, filed concurrently herewith, describes another method of post-crosslinking the polymers. This method reacts silanol modified organic polymers prepared in a manner similar to that described in U.S. Pat. No. 3,729,438 with a hydrolyzable silane containing at least one group which is capable of further reaction, such as 3-acryloxypropylmethyldimethoxysilane, to form a functionalized organic emulsion polymer. If a photoinitiator is added to this functionalized organic emulsion polymer, it will crosslink in the presence of ultraviolet radiation.
Heretofore a method has not been disclosed for the preparation of aqueous emulsions which crosslink into silicon modified organic compositions by use of the Michael addition reaction. In preferred embodiments, this crosslinking may occur after application in the dry state. The resulting films are useful as coatings, paints and sealants.
An objective of this invention is to prepare aqueous emulsions which crosslink by a Michael addition reaction in either the wet or dry state.
Another objective of this invention is to prepare aqueous emulsions which yield crosslinked silicon modified organic compositions upon the removal of water and do not require an external stimulus in order to effect crosslinking.